1. Field of the Invention
The present invention relates to a circuit board and particularly to a circuit board using a metal substrate, having excellent heat dissipating characteristics enabling high-density mounting of semiconductor devices or chip components for an integrated circuit (IC), a large-scale integrated circuit (LSI) or the like, as well as a method of manufacturing thereof.
2. Description of the Prior Art
There is an increasing demand, these days, for high-density assembly of electronic components in various fields of electronic equipment. In consequence, it becomes important how to efficiently remove heat generated from the electronic components mounted on a circuit board.
Organic high polymer materials such as paper phenol, glass epoxy or glass polyimide or inorganic materials such as alumina are conventionally used as circuit board materials. However, any of those materials has insufficient thermal conductivity, which causes limitations in coping with increase of a calorific density caused by heat emitted from the electronic components. A circuit board using, as a substrate, a metallic plate having excellent heat resistance and heat dissipation characteristics has been developed to solve this problem.
FIG. 1 is a sectional view showing a conventional circuit board using a metal substrate on which a semiconductor device is mounted. Referring to FIG. 1, an adhesive material 2 to be an insulating layer is placed on a metal substrate 1. A conductor pattern 3 of copper foil is formed on the adhesive material 2. A junction pad 4 comprises a die pad 42 and bonding pads 41 and it is selectively formed on the adhesive material 2. Resist 5 is formed on the conductor pattern 3 only in portions where a surface metallizing process does not need to be applied. An organic high polymer material such as epoxy or polyimide is used as the adhesive material 2. Various techniques are applied conventionally to the adhesive material 2. For example, inorganic filler is contained in the adhesive material 2 to improve thermal conductivity of the adhesive material 2, or prepreg obtained by permeation of semihard resin in glass fabric is used to make the thickness uniform and to suppress occurrence of voids. The thickness of the adhesive material 2 needs to be about 20 to 100 .mu.m in order to ensure electric insulation between the metal substrate 1 and the conductor pattern 3.
The metal substrate 1 has preferably as large a coefficient of thermal conductivity as possible and copper, copper alloy, aluminum, aluminum alloy, iron, iron alloy or the like of 0.5 to 3 mm in thickness is used. The circuit pattern including the conductor pattern 3 and the junction pad 4 is obtained by a process of forming resist pattern on the copper foil and etching the copper foil. A surface metallizing process suitable for a junction method of a semiconductor device to be mounted thereon, for example, a metallizing process of Au, Ni, Ni/Au or soft solder is applied to the die pad 42 and the bonding pads 41 of the junction pad 4, as required. A method of forming a junction of an electronic component 18 such as a semiconductor device to the die pad 42 includes an adhesion method using a conductive or non-conductive material, an alloy junction method using soft solder or Au-Si, and the like. Connection between the electronic component 18 and the bonding pads 41 is made by lead wires 19 through a metallized layer 6 formed on the bonding pads 41.
FIG. 2 is a sectional view showing a conventional board including connection layers on both sides of the metal substrate 1. Copper foil layers 7 are placed on both surfaces of the metal substrate 1 perforated in advance through the adhesive material 2 that acts as an insulating layer, and those are integrally formed as a unitary body. Through holes 8 are formed in the perforated portion filled with the adhesive material 2, penetrating the respective copper foil layers 7. A conductor layer 10 is selectively formed on the adhesive material 2 through the copper foil 7. A heat emitting electronic component 18 has lead pins 21 inserted in the through holes 8 corresponding thereto and it is soldered by a flow soldering method or the like so as to be mounted fixedly on the circuit board.
FIG. 3 is a sectional view showing a conventional board including multilayer connection layers on the metal substrate 1. A multilayer substrate 9 having conductor layers 10 including inner layers and outer layers as well as through holes 8 is placed on the metal substrate 1 by means of the adhesive material 2, thereby to form a unitary body. An electronic component 18 (such as a flat pack IC or a chip carrier) to be mounted on the surface is connected to the surface of the multilayer substrate 9 through the conductor layers 10 by means of solder 22.
A circuit board technology using a metal substrate is described for example in "Electronic Materials", 1986 Oct. issue, pp. 72-77. Particularly, for a circuit board using through holes, this journal indicates an example in which an aluminum substrate is used as a substrate where light emitting diodes (LEDs) are mounted, thereby to solve the problem of insufficient heat dissipation in the case of high-density mounting of LEDs.
In the conventional circuit board using the metal substrate, a connection circuit is formed on the metal substrate through an insulating layer having lower thermal conductivity as described above. Consequently, heat dissipation from a heat generating component mounted on the pad to the metal substrate is insufficient and if IC or LSI devices having a large calorific quantity are mounted with high density, the heat dissipation characteristic is insufficient and the temperature in the devices increases excessively, causing deterioration in the properties of the devices and problems in the associated equipment. In addition, in such a case, the quality of the adhesive material of the insulating layer is deteriorated due to the temperature rise and the electric insulation thereof is lowered. Further, thermal stress is caused by a difference of thermal expansion coefficients of the materials constituting the substrate as a result of temperature changes in the devices due to turn-on and turn-off of the equipment, resulting in lowering of adhesion intensity or detachment between the insulating layer (the adhesive material) and the metal substrate, or between the insulating layer (the adhesive material) and the copper foil.